The present invention relates to semiconductor laser diode devices, and more particularly to a semiconductor laser diode device suitable for use as a light source in optical fiber communication and a method of fabrication thereof.
There are two structural types of distributed feedback semiconductor laser diode (DFB-LD) devices; anti-reflection/anti-reflection (AR/AR) coated type, and anti-reflection/high-reflection (AR/HR) coated type. An AR/AR DFB-LD device is configured to have both front and rear facets thereof coated for anti-reflection as shown in FIG. 10, and an AR/HR DFB-LD device is configured to have a front facet thereof coated for anti-reflection and a rear facet thereof coated for high reflection as shown in FIG. 11.
FIG. 10 is a configuration diagram of a typical conventional AR/AR DFB-LD device, schematically illustrating a longitudinal cross-section parallel to the optical waveguide axial direction thereof. FIG. 11 is a configuration diagram of a typical conventional AR/HR DFB-LD device, schematically illustrating a longitudinal cross-section parallel to the optical waveguide axial direction thereof. As shown in FIGS. 10 and 11, each DFB-LD device has an optical waveguide including a grating structure (diffraction grating structure) in which a periodic grating 1 is provided. In the grating structure, a period of the grating 1 is set to be “λ/2” (λ: indicates a wavelength in the optical waveguide). In most cases, a phase shift part 4 having an optical path length of λ/4 is disposed at a predetermined position in the grating structure. Thus, single-wavelength laser oscillation is performed. The term “optical path length” as used herein denotes a value of “(Refractive index of optical waveguide)×Distance”.
The AR/AR DFB LD device is favorable in that the fabrication yield regarding single-mode wavelength oscillation is relatively high. Contrastingly, the AR/HR DFB-LD device is favorable in that the forward/backward optical output ratio (=ratio of forward optical power to backward optical power) is relatively high to provide a superior level of forward optical output. While the AR/AR DFB-LD device has a forward/backward optical output ratio of approximately 1.5 at the maximum, the AR/HR DFB-LD device has a forward/backward optical output ratio of 4 or higher. For example, in Patent Document 1 indicated below, there is disclosed an AR/HR DFB-LD arrangement wherein a resonator length L is 600 μm, and two λ/4 phase shift parts are disposed at positions of 440 μm and 500 μm from the rear facet position respectively to provide “κL=3” so that an output of optical power from the front facet will be eight times higher than optical power at the rear facet. In Patent Document 2 indicated below, there is disclosed an AR/HR DFB-LD arrangement wherein a λ/4 phase shift part is disposed at a position where the intensities of leftward and rightward reflected radiations are equal to each other.
However, it is known that, in practical fabrication of AR/HR DFB-LD devices, defects of poor single-mode oscillation are encountered at a rejection rate of approximately 30% due to adverse effects of improper phasing in a grating 1 caused by high reflection at the rear facet position.
As shown in FIGS. 10 and 11, in the formation of a grating 1 for a DFB-LD device, a periodic structure patterned over an InP substrate 2 is buried with an InGaAsP guide layer 3. An optical coupling coefficient κ, which is expressed by the following equation (1), is used as an index representing the degree of periodic variation in refractive index in an optical waveguide corresponding to the grating 1, i.e., the degree of longitudinal mode fixation of waveguided light with respect to periodic variation in refractive index. Thus, the optical coupling coefficient κ denotes an intensity level of mutual action of a DFB-LD grating structure and a light beam.κ=(n1−n2)π/λ0  (1)
In the equation (1),
“λ0” indicates a wavelength of laser oscillation light in a vacuum,
“n1” indicates an equivalent refractive index at a grating valley in the optical waveguide concerned, and
“n2” indicates an equivalent refractive index at a grating peak in the optical waveguide concerned.
In cases where DFB-LD devices are produced through ordinary fabrication processes, the entire optical waveguide of each DFB-LD device has a uniform value of optical coupling coefficient κ.
In contrast, significantly advantageous effects can be attained by providing different optical coupling coefficient values κ in different regions of an optical waveguide.
For example, if a configuration having a relatively large value of optical coupling coefficient κ only in a device rear region of an optical waveguide is formed and applied to AR/AR DFB-LD design, it becomes possible to produce an AR/AR DFB-LD device capable of providing a forward/backward optical output ratio as high as that of an AR/HR DFB-LD device while ensuring satisfactory stability of single-mode oscillation that is characteristic of AR/AR DFB-LD design. In Patent Document 3 indicated below, there is disclosed an AR/HR DFB-LD configuration in which a relatively large value of optical coupling coefficient κ is provided in a device rear region by using a different etched grating pattern (depth) therein. Patent Document 4 indicated below discloses a configuration in which a value of optical coupling coefficient κ varies gradually from a device front position to a device rear position (different heights of grating are provided), along with the description of a method of fabrication thereof. Patent Document 5 indicated below discloses a configuration in which a guide layer is provided with a buried grating structure, a partial composition of a rear region of the guide layer being arranged to be different from the composition of a front region of the guide layer (the rear region has a composition for a wavelength longer than that in the front region), and the depth of a grating structure part buried with the guide layer in the front region being arranged to be different from that in the rear region. The disclosure of a method for fabricating the above configuration is also included in the Patent Document 5.
Further, as another example for attaining advantageous effects by providing different optical coupling coefficient values κ in different optical waveguide regions, it has been proposed to form a configuration in which a value of optical coupling coefficient κ in the vicinity of a λ/4 phase shift part is smaller than that in the other regions.
Generally, at the time of laser oscillation in a DFB-LD device, the level of optical density becomes higher in the vicinity of a λ/4 phase shift part. This is one of the causes of occurrence of wavelength chirping in modulation drive. The phenomenon “wavelength chirping” is an undesirable condition of wavelength variations on the order of Δv to several GHz, which brings about an adverse effect on optical signal communication such as a decrease in transmission limit distance. Hence, it is desirable to reduce the degree of wavelength chirping wherever possible.
It is already well known that the concentration of optical density in the vicinity of a λ/4 phase shift part can be alleviated by providing a smaller value of optical coupling coefficient κ in the vicinity of λ/4 phase shift part than that in the other regions. Thus, the degree of wavelength chirping can be reduced advantageously. For example, in Patent Document 6, there is disclosed a technique wherein the degree of wavelength variations in pulse modulation can be minimized without impairing the stability of single-longitudinal-mode oscillation under the conditions that the value of reflectivity at each of both front and rear facets is relatively small “=0.3%”, the value of κL (L: resonator length) is in a range of “0.8<κL<3.0” except a position in the vicinity of 1.3, and the value of phase shift Δθ in a discontinuous phase part at the center area of a resonator is in a range of “nπ+0.5π<Δθ<nπ+0.75π (λ/4<Δθ<3λ/8)”.
In Patent Document 9 indicated below, there is disclosed a DFB-LD device wherein a plurality of grating parts having mutually different pitches are disposed along the axial direction of an active layer.
The degree of freedom in DFB-LD design can be increased advantageously by using a technique for providing different optical coupling coefficient values κ in different optical waveguide regions as mentioned above.